This has been requested for years. I've linked one old thread - there are others. I'd say the Articulate Devs have long ago binned all requests for specifying an accurate corner radius numerically and have no intention of ever implementing this (would love to be proved wrong)
https://community.articulate.com/discussions/articulate-storyline/corner-radius
As a long-time Storyline user, I've waited; mostly, but not always, patiently, for sensible, productive features like this to be implemented.
As I wait, I'm aware I no longer look anything like my user profile photo. My hair has thinned and turned grey, the once bright smile has flattened, skin tones have sallowed. Still I wait...